As a method of switching a channel in the conventional type of cordless telephone system, there have been put into practical use a method in which a signal vs noise ratio during communications is detected and channel switching is executed depending on a time ratio lower than a specified threshold value, and a method in which a received signal input (level) is detected and channel switching is similarly executed depending on a time ratio lower than a specified value, and furthermore there has been proposed a method in which channel switching is executed during call depending both a relation between a signal vs noise ratio and a specified threshold value and a relation between a received signal level and a specified threshold value in mobile radio communication. In addition, also a method has been proposed in which a ratio of communication level vs an interference wave level in a calling channel which is a channel in service is measured and a channel is switched when the measured value is not more than a specified value.
Also in U.S. Pat. No. 4,783,844 is disclosed a system in which a specific ID code assigned to each device is used for communication and channel switching is executed according to whether the ID code has correctly been received or not. Namely in this system, when the ID code is not received correctly, the channel in service is switched depending on the detection signal.
In a mobile radio communication system such as a cordless telephone system, however, detection of disturbance by an interference wave is very difficult, and also in the systems as described above, if strong interference occurs, transaction of control data required for an operation to switch a channel becomes impossible, which in turn makes it difficult to continue communication.
In the channel switching method disclosed in U.S. Pat. No. 4,783,844 described above, it is necessary to set up a special ID code for only switching a channel, and also it is necessary to always receive or send the ID code together with an voice signal. As a result, it is necessary to accurately separate an voice signal using a relatively high frequency from a data signal using a relatively low frequency, and excessive work load is disadvantageously loaded to a modulating section executing the separating operation above or a demodulating section.